The invention relates to a tire building drum with a circumferential surface with a variable radius.
U.S. Pat. No. 3,932,256 A discloses a tire building drum having a large number of segments, which can move radially to expand and to contract the drum. The drum is further provided with cover plates for spanning the gaps between the segments in the expanded state of the drum. Each cover plate has a fixed longitudinal edge rigidly affixed to one of said segments and a free longitudinal edge overlying the other of said segments, the plate having an arched contour that, when free from an externally applied load, is support by one of the pair of segments only. The other of the pair of segments has cover plate support surface that is declined inwardly to underlie and to support said free edge when a load is applied to said cover plate.
JP 2009-274392 A discloses a similar expandable tire building drum, also having a plurality of segments and cover plates for spanning the gaps between the segments. Each cover plate is coupled to one of the segments via a spring loaded hinge, that is arranged to bias the cover plates onto a recessed cover plate support surface of an adjacent one of the segments. The cover plates are given a warp such that the free ends are always in contact with the cover plate support surface.
It is noted that in both the aforementioned prior arts, the cover plates and the segments are optimally adjusted for the radius of the circumference of the drum in the contracted state. However, in the expanded state, the circumference of the drum has a larger radius. As a result, the circumference of the drum will show a plurality of dips at the location where the cover plates rest on the adjacent segments. In addition, when the drum is expanded, the segments move apart and each cover plate moves in the circumferential direction of the drum with the segment to which it is fixed, thereby at least partly exposing the underlying cover plate support surface of the adjacent segment. As the exposed cover plate support surface is declined or recessed inwardly with respect to the base surface of the segment to accommodate the cover plate in the contracted state, a dip or waving pattern in the circumferential surface is created at the location of the cover plate support surface in the expanded state. These dips or waving patterns seriously affect the consistency of the tire component supported thereon, which ultimately affects the overall quality of the tire.
U.S. Pat. No. 6,013,147 A discloses an expandable belt and tread drum with an annular shell comprising a plurality of circumferentially spaced segments. Each segment is provided with an intermediate section and two adjacent side sections. The side sections are provided with fingers and mating slots that mesh with the side sections of adjacent segments. The side sections do not cover, overlap or support each other in the radial direction of the drum and therefore do not form a closed circumferential surface. The gaps between the side sections can seriously deform or even damage the tire components during expansion of the drum.
The side sections of U.S. Pat. No. 6,013,147 A are pivotally attached to the intermediate section for hinged movement relative thereto. Each segment further comprises guide plates which are rigidly mounted to the intermediate section and which extend in opposite directions to the side sections of adjacent segments. The guide plates are provided with elongated slots which are engaged by respective pins of the side sections of the adjacent segments. The shape of the elongated slots defines the pivoting behavior of the side sections of adjacent segments. When the segments move apart in the circumferential direction during the expansion of the drum, the elongated slots guide the side sections of the respective adjacent segments. The pivoting is thus a direct result of the relative spacing between the segments in the circumferential direction. Although U.S. Pat. No. 6,013,147 A provides a solution for pivoting meshing side sections of adjacent segments, it does not provide a solution for optimizing the consistency of a circumferential surface formed by segments and cover plates which are radially supported on the segments.
It is an object of the present invention to provide a tire building drum with a circumferential surface with a variable radius, wherein the consistency of the circumferential surface can be improved.